Hardiness Zones

Hardiness Zones

The hardiness zones are informative: the extremes of winter cold are a major determinant of whether a plant species can be cultivated outdoors at a particular location; however, the USDA hardiness zones have a number of drawbacks if used without supplementary information.

The hardiness zones are informative: the extremes of winter cold are a major determinant of whether a plant species can be cultivated outdoors at a particular location; however, the USDA hardiness zones have a number of drawbacks if used without supplementary information.

The zones do not incorporate summer heat levels into the zone determination; thus sites which may have the same mean winter minima, but markedly different summer temperatures, will be accorded the same hardiness zone. An extreme example of this phenomenon is seen when comparing the Shetland Islands and southern Alabama, which are both on the boundary of zones 8 and 9 and share the same winter minima, but little else in their climates. In summer, the humid subtropical climate of Alabama is about 20 degrees Celsius hotter than the temperate maritime climate of Shetland, and there are few similar plants that can be grown at both locations. Due to its maritime climate, the UK is in AHS Heat Zone 2 (having 1 to 8 days hotter than 30 degrees Celsius) according to the AHS (American Horticultural Society), whereas Alabama is in Zones 7 to 9 (61 to 150 days hotter than 30 degrees Celsius). Users need to combine the hardiness zone with the heat zone to gain greater understanding of what can be grown in a particular location.

Another issue is that the hardiness zones do not take into account the reliability of the snow cover. Snow acts as an insulator against extreme cold, protecting the root system of hibernating plants. If the snow cover is reliable, the actual temperature to which the roots are exposed will not be as low as the hardiness zone number would indicate. As an example, Quebec City in Canada is located in zone 4, but can rely on an important snow cover every year, making it possible to cultivate plants normally rated for zones 5 or 6. But, in Montreal, located to the southwest in zone 5, it is sometimes difficult to cultivate plants adapted to the zone because of the unreliable snow cover.

Other factors that affect plant survival, though not considered in hardiness zones, are soil moisture, humidity, the number of days of frost, and the risk of a rare catastrophic cold snap. Some risk evaluation – the probability of getting a particularly severe low temperature – often would be more useful than just the average conditions.

Lastly, many plants may survive in a locality but will not flower if the day length is insufficient or if they require vernalization (a particular duration of low temperature). With annuals, the time of planting can often be adjusted to allow growth beyond their normal geographical range.

An alternative means of describing plant hardiness is to use "indicator plants" (the USDA also publishes a list of these to go with the hardiness zone map). In this method, common plants with known limits to their range are used.

Gardening books are available that provide more information on climate zones. For example, Sunset Books (associated with Sunset Magazine) publishes a series that break up climate zones more finely than the USDA zones. They identify 45 distinct zones in the US, incorporating ranges of temperatures in all seasons, precipitation, wind patterns, elevation, and length and structure of the growing season.

Zone

untere Grenze

obere Grenze

1

- - -

unter -45,5° C

2

- 45,5° C

- 40,1° C

3

- 40,0° C

- 34,5° C

4

- 34,4° C

- 28,9° C

5

- 28,8° C

- 23,4° C

6

- 23,3° C

- 17,8° C

7

- 17,7° C

- 12,3° C

8

- 12,2° C

- 6,7° C

9

- 6,6° C

- 1,2° C

10

- 1,1° C

+ 4,4° C

11

+ 4,5° C

+ 10,0° C

12

ab 10,1° C

- - -

European cities

The table herein provides hardiness zone data for some European cities (based on climatological data):

City

Zone

City

Zone

Amsterdam, The Netherlands

8

Antwerp, Belgium

8

Belfast, Northern Ireland

9

Berlin, Germany

7a

Bratislava, Slovakia

7a

Bucharest, Romania

6a

Cardiff, Wales

9

Catania, Italy

10

Copenhagen, Denmark

7a

Cork, Ireland

9b

Dublin, Ireland

9

Düsseldorf, Germany

8

Edinburgh, Scotland

8

Gdańsk, Poland

7

Glasgow, Scotland

9a

Hamburg, Germany

7

Helsinki, Finland

5b

Istanbul, Turkey

8b

Kaliningrad, Russia

6

Kiev, Ukraine

5

Kraków, Poland

6

Lisbon, Portugal

10a

Ljubljana, Slovenia

7

London, England

9a

Madrid, Spain

8-9

Málaga, Spain

10b

Marseille, France

9

Milan, Italy

7-8

Minsk, Belarus

5а

Moscow, Russia

4

Munich, Germany

6

Murmansk, Russia

5

Nicosia, Cyprus

10b

Oslo, Norway

6

Simferopol, Ukraine

7a

Palma, Spain

10

Paris, France

8a

Poznań, Poland

7a

Prague, Czech Republic

7a

Reykjavík, Iceland

7

Riga, Latvia

6

Rome, Italy

9b

Rovaniemi, Finland

4

Saint Petersburg, Russia

5a

Sarajevo, Bosnia and Herzegovina

7

Santander, Spain

10a

Simrishamn, Sweden

8a

Sochi, Russia

9

Sofia, Bulgaria

6

Stockholm, Sweden

6a

Strasbourg, France

7

Tallinn, Estonia

5

Tuapse, Russia

8

Tórshavn, Faroe Islands

7-8

Tromsø, Norway

7

Trondheim, Norway

6

Umeå, Sweden

5

Vienna, Austria

6-7

Vilnius, Lithuania

5

Vorkuta, Russia

2

Valletta, Malta

10b

Warsaw, Poland

6

Zürich, Switzerland

7

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Product: Hardiness Zones

Description: Hardiness Zones
The hardiness zones are informative: the extremes of winter cold are a major determinant of whether a plant species can be cultivated outdoors at a particular location; however, the USDA hardiness zones have a number of drawbacks if used without supplementary information.
The zones do not incorporate summer

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